1. Field of the Invention
The invention concerns a temperature compensated oxygen sensor of the resistive type, for the measurement of relative concentrations of fluid reactive species.
It pertains, notably, to a device capable of producing a signal for the control, in closed loop, of an air/fuel mixture such as the mixtures that feed combustion machines of the internal combustion engine type, or boilers, in particular boilers with forced air circulation.
2. Description of the Prior Art
In recent types of internal combustion engines, particularly in automobile engines with automatic ignition, a marked trend can be seen towards the precise control of the carburation, so as to increase efficiency and performance, on the one hand, and reduce the polluting exhaust emissions on the other hand. In many cases, it is desired to feed the motor with a stoichiometric air/fuel mixture and, in practice, there already are regulation systems that keep this mixture in the stoichiometric state by using an oxygen sensor placed in the exhaust muffler. This sensor delivers a signal representing the state of the mixture given to the cylinders, in indicating whether the mixture in question is rich or lean.
Thus, most of the already existing oxygen sensors deliver a signal of about one volt when the mixture is rich, and a signal of about 50-100 mV when the mixture is lean. The electronic carburation-regulating system therefore works by comparing the value of the output signal given by the sensor, with a voltage reference that is generally located around 500 mV. Under these conditions, if the output voltage from the sensor is greater than 500 mV, which corresponds to a situation where a rich mixture feeds the cylinders, the electronic system for the regulation and servo-control of the engine corrects the fuel injection parameters so as to increase the air/gasoline ratio and, when the output voltage from the sensor is smaller than 500 mV, the electronic system reduces the air/gasoline ratio in such a way that, on the whole, the mixture is always exactly stoichiometric.
In practice, for the vehicles fitted out with trifunctional catalytic mufflers, the carburation has to be regulated by means of an oxygen probe. The standard mechanical settings do not allow the mixture to be kept in a composition range that is narrow enough to be compatible with high efficiency of the catalytic muffler in question.
Two distinct types of oxygen sensors are currently being fabricated or designed: galvanic (or voltaic) sensors and resistive sensors.
Galvanic sensors, which are the most widespread ones, work on the principle of the concentration cell. A glove finger made of stabilized zirconia has its external surface in contact with the exhaust gases, while its internal surface is in contact with an oxygenated reference mixture which is generally air. The internal and external surfaces are provided with electrodes, made of platinum for example, which are used, firstly, to measure the electrochemical potential and, secondly, for the external surface, to catalyze the end of the combustion of the gaseous exhaust mixture.
Various descriptions of this type of sensor will be found in the literature on the subject (H. Dueker, K. H. Friese, W. D. Haecker, Society of Automotive Engineers 1975, paper 750223; H. Y. Gruber, H. M. Wiedenmann, Society of Automotive Engineers, 1980, paper 800017; E. M. Logothetis in Advances in Ceramics, edited by A. M. Heur and L. W. Hobbs, Vol. 3, Science and Technology of Zirconia (American Ceramic Society, Columbus Ohio, 1981, p. 388 etc.).
Resistive sensors work according to a different principle, in that they exploit the properties of variation in resistivity, of certain oxides of transition metals, as a function of the ambient partial pressure of oxygen. Certain allotropic varieties of the oxides of transition metals (for example: Ti, Nb, Ce . . .) have, in effect, a resistivity of the form: ##EQU1## where: .rho. o is a constant
.rho. o.sub.2 is the ambient partial pressure of oxygen, PA1 k is the Boltzmann constant PA1 T is the absolute temperature in Kelvin PA1 W is an energy related to the creation of a localized flaw in the crystal lattice of the oxide considered (for example, oxygen defect or interstitial cation) PA1 .alpha. is a constant (1/4 or 1/6 as the case may be) which is also related to the equilibrium of the flaws in the crystal lattice of the oxide considered. PA1 (i) an element sensitive to the partial pressure of oxygen; PA1 (ii) a temperature compensation thermistor; PA1 (iii) if necessary, a resistor enabling the value of the thermistor to be adjusted; PA1 (iv) a heating resistor; PA1 (v) connection means. PA1 at least one sensitive element, the resistivity of which is sensitive to an excess of one of the reactive species with respect to a determined stoichiometry; PA1 at least one thermistor undergoing temperature variations proportionate to the temperature variations undergone by the sensitive element, this thermistor being mounted as a resistance bridge with the sensitive element; PA1 a heating resistor heating the sensitive element and the thermistor and fixing a minimum temperature threshold of operation.
and
It is thus seen that, if the partial pressure of oxygen in an exhaust mixture is brought to its thermodynamic equilibrium value (see FIG. 1), a resistor made out of the rutile form of titanium oxide, for example, (o.sub.TiO2 =1/4) will exhibit a variation in the range of practically four magnitudes when the air/fuel mixture feeding the cylinders touches the stoichiometric point.
It is therefore this property that we shall seek to exploit in the use of resistive type oxygen sensors for the purpose of controlling an air/fuel mixture at the stoichiometric point.
The oxygen sensors based on titanium oxide (rutile form) have been known for about the past 12 years (T. Y. Tien, H. L. Standler, E. F. Gibbons, P. J. Zacmandis, Ceramic Bull., 54, 280, 1975; E. M. Logothetis, Ceramic Eng. and Sci. Proc., 8th Automotive Mat. Conf., 1980; M. J. Esper, E. M. Logothetis, J. C. Chu, Society of Automotive Engineers, 1979, paper 790140; K. Saji, H. Takahashi, K. Kondo, T. Takeuchi, I. Igarashi, Proc. of the International Meeting on Chemical Sensors, Edited by T. Seiyama and Coll., Elsevier 1983, p. 171 etc.)
These sensors take the form of TiO.sub.2 disks, in which platinum contact wires have been buried, enabling the value of the resistance of the disk in question to be measured as a function of the partial pressure of oxygen. The drawback of these devices comes from the variations in temperature of the exhaust gases between 300.degree. C. and 900.degree. C. depending on the engine speed. In effect, if we refer to the formula (1) giving the resistivity of titanium oxide, for example, we see that a variation in temperature of 300.degree. C. to 900.degree. C. also produces a variation in resistivity equal to about four magnitudes.
A certain form of temperature compensation must therefore be introduced in order to make it possible to establish a clear distinction between the variations in resistivity due to the variation in the partial pressure of oxygen and the variations in resistivity due to the temperature variation.
Recently, sensors of an "advanced type" have appeared. For these sensors, the techniques of microelectronics and, particularly, silk screen process techniques are used to fabricate sensitive elements made of titanium oxide (W. J. Kaiser, E. M. Logothetis, Society of Automotive Engineers 1983, paper 830167; H. Kondo, H. Takakashi, T. Takeuchi, I. Igarashi, Proc. of the 3rd Sensor Symposium 1983, p. 185; D. S. Howart, A. L. Micheli, Society of Automotive Engineers, 1984, p. 840140; J. L. Pfeifer, T. A. Libsch, H. P. Werthermer, Society of Automotive Engineers, 1985, paper 850381; A. Takami, M. Matsuura, T. Sekiya, T. Okawa, Y. Watanabe, Society of Automotive Engineers, 1985, paper 850381).
The sensors thus described are made with inks or pastes having the desired oxide (TiO.sub.2, CeO.sub.2, Nb.sub.2 O.sub.3 etc. . . ) in the form of a suspension in an organic medium suited to a silk screen process type of forming operation. This ink is then deposited on a substrate that is a good electrical insulator and is chemically inert, of the alumina type. It is deposited according to a determined geometry and undergoes appropriate heat treatment enabling the deposit to be given the desired characteristics, particularly as regards porosity.
However, the working point, in temperature, is chosen by means of a heating resistor placed on the rear face of the alumina substrate and making it possible to keep the sensitive element in a relatively narrow range of temperature (of the order of 100.degree. C.) irrespectively of the exhaust gases.
The invention pertains to an advanced type of sensor that integrates the following elements on one and the same chemically inert, electrically insulating, ceramic substrate: